This invention relates, generally, to neuronal interface devices. More particularly, it relates to such a device that replaces silicon with cubic silicon carbide.
Silicon is known to be used in the manufacturing of bipolar junction transistors, junction field effect transistors, and metal insulator transistor devices.
Field effect devices constructed from silicon and silicon dioxide have been used to interact with neurons by receiving and sending signals. Field effect devices, unlike electrode based devices, use a high electrical impedance to influence the electrochemical environment within the body. Unfortunately, field effect devices are ineffective for use over long periods of time for several reasons. One known disadvantage to field effective devices is that sodium ions may diffuse through the oxide and create a fixed positive charge at the interface, which an undesirable result. More importantly, silicon and silicon oxide have been proven to be non-biocompatible for use as an implantable neural device.
The majority of semiconductors which could replace silicon are also known to be toxic. However neural devices must, by necessity, be implanted permanently so they must be very robust and, to date, a semiconductor-based neural device, such as a brain-machine interface (BMI), has failed to perform adequately for long-term chronic implantations.
The field of implantable brain machine interfaces (BMIs), also known as brain computer interfaces (BCIs), is a burgeoning technology developed by engineers, neurologists, and computer scientists that has the potential to offer hope and possible cures for many problems associated with the central nervous system.
A BMI and BCI device allows reception of electrical pulses generated by neurons and transfer of the electrical pulses to external devices where they may be used as control signals for the nervous system. BMI devices known in the art utilize penetrating, invasive interfaces directly accessing the signals from nearby neurons. The proximity of the BMI device to these neurons also allows the direct stimulation of these signals, known as action potentials, and can thereby create true closed loop control systems.
Implantable devices, such as BMIs, inherently possess a larger content and quality of information, and therefore can easily outperform other interface techniques such as electroencephalography (EEG) and electrocortiography (ECG). Although implantable devices allow for closed loop control and sensory feedback, and the information they acquire can control complicated BMI systems, these systems still experience major difficulties which are blocking their widespread usage.
Conventional implantable neural interface devices use four well-established architectures: 1) Macroelectrode wire implants; 2) Microelectrode wire arrays; 3) The Utah intracortical electrode array (UIEA); and 4) The Michigan planar microelectrode. These devices operate through conductive electrical transducers (i.e., electrodes) which interface and react with the electrolytic fluid surrounding the cells in the body. The ionic and electron changes, movement within and surrounding electrodes and electrical activity of the cells allow signal transport in both directions. This interaction can either be a capacitive interaction through the formation of a Helmholtz double layer, or it can be Faradaic which involves oxidation or reduction chemistry at the electrode surface. Capacitive charge transfer does not generate any chemical reaction, but can be limited by geometrical surface area. On the other hand, Faradaic reactions at the electrode surface can provide high levels of charge transfer, but the high transfer rates can lead to irreversible processes which damage both the electrode and surrounding cells.
Electrodes, both in vitro and in vivo, have displayed many problems that can lead to the reduced reliability which has been observed with implantable neural devices. Faradaic reactions can electrolyze water, produce pH changes, and produce gasses and reactive ionic species. Another consideration is that electrode polarization is over two (2) times greater in vivo, which is thought to be the result of protein and molecule absorption onto the electrode surface as well as an increased ionic resistance within tissue surrounding the implant. The surface absorption of various molecules can lead to an increase in the impedance of the electrode, which reduces the signal to noise ratio and decreases the high frequency response.
The geometrical surface area of the electrode also has a large effect on the ability of an electrode to withstand charge transfer changes, and larger electrodes are generally better able to withstand the charge densities required to stimulate neural responses without deterioration. It has been deduced that maintaining a charge balance during stimulation by using biphasic wave forms helps reduce damage to the electrode and surrounding tissue.
An alternative to electrode interaction with neurons was developed by P. Fromherz from the Max-Planck-Institute. In 1985 he presented the idea that silicon (Si) devices and brains could be directly coupled without electrode transducers. He has since shown that Si metal-oxide-semiconductor capacitors (MOScaps) can stimulate neuronal membrane depolarization, and Si metal-oxide-semiconductor field effect transistors (MOSFET) can receive electrical signals from neurons. The major advantage of MOS, and the related device, metal-insulator-semiconductor (MIS), is that they interact with the cell using capacitive/electric field effects. The MIS devices avoid the material and tissue damaging Faradaic reactions which are encountered with electrode transducers. As a further bonus, the device operation is based around electric fields generated on the opposite sides of an insulating material and, due to this action, they are not as susceptible to increases in tissue impedance, as they are constructed with highly insulating, high impedance, capacitive functionality.
Another advantage of FET devices is that they can be interlinked into arrays by combining common connections, thereby reducing the amount of feed-through wiring required and allowing many more active interaction areas on the neural interface, with current art showing 16,384 contacts per square millimeter. Finally, unlike electrode transducer devices, stimulation of neuronal action potentials with FET capacitor devices maintains the charge balance through a single monophasic pulse which dissipates naturally through RC parasitic losses when the signal is removed.
Initially, Fromherz et al. used MOSFET transistors with no gate metal to receive action potential signals from an electrically active neuron. This interaction occurs because the ionic metals in the extracellular solution form the gate metal charge at the oxide interface. This interaction creates a series capacitive coupling between the gate oxide and the cell membrane. The two (2) capacitors have a parallel parasitic resistance which Fromherz et al. labels the transductive extracellular potential, but it has also been referred to as the sealing resistance by others who were investigating the electrical model for electrode transducers.
Unlike the passive operation of an electrode, signals are intercepted by the MOSFET through a different method. The MOSFET is driven into saturation, or active mode, (Vsd>|Vgs−Vth|, Vs=Vb=VDD, and VDD>>Vsd) which provides a constant current flow through the device. In saturation, the MOSFET drain current is directly controlled by fluctuations in the gate voltage. The extracellular ionic flux generated by a neuronal action potential changes ionic and displacement currents within the transductive extracellular potential, which changes the charge on the gate oxide, and then is transferred to the depletion region within the gate and modulates the drain current within the MOSFET device.
Stimulation of an action potential in a neuron can also be achieved through a similar field effect device, the metal-oxide-semiconductor capacitor, or MOScap. Fromherz et al. show that this device uses the same principles that allow the transistor device to function. A buried conductive channel under an oxide on a MOScap is stimulated with a voltage which gives rise to ionic and displacement currents on the opposite side of the insulator. The fields generated at the oxide surface modulate the transductive extracellular voltage to depolarize the membrane, and if this voltage was greater than the threshold voltage of the neuron, a neuronal action potential is produced. The combination of the MOScap and MOSFET devices allow bi-directional signaling, and can be used with digital electronic processing to create bridges between different neuronal cells.
Fromherz et al. have developed this technology in Si to the point where they can interface with complex neural tissue, single neuronal cells, voltage controlled ionic gates in the cellular membrane, and even the chemicals that pass between active synapses. These capabilities place this device far ahead of the electrode devices, but there are many problems that this type of device must overcome if it will have resiliency in vivo.
Reliability of implanted field effect devices within the body environment is one of the major issues that prevents this technology from being used outside neurology and electrophysiology laboratories. One of the greatest problems with Si-based field effect devices is their susceptibility to contamination from metallic ions, such as sodium (Na), potassium (K), and iron (Fe). This problem is well known in the semiconductor industry, and is one of the driving forces behind the development of the cleanroom for processing electronic devices. Metallic ions diffuse through the insulating silicon dioxide (SiO2) layer and create fixed positive charges at the interface, which degrades the reliability and functionality of field effect devices. These devices are in direct contact and interacting with the extracellular fluid, or cerebral spinal fluid, which contains copious amounts of Na+, K+, Cl−, Ca+2, and Mg+2 ions.
Another important consideration is that both Si and SiO2 are not necessarily biocompatible, and may provide negative biological effects, like cancer and Alzheimer's disease, when used for applications in implantable brain machine interfaces. Many modern semiconductors which could be substituted for Si possess many of the same problems with respect to immunoreactivity, or these materials produce toxic byproducts due to their atomic constituents (i.e. gallium, arsenic, indium, antimony, etc.).
However, in view of the art considered as a whole at the time the present invention was made, it was not obvious to those of ordinary skill in the art how the art of neuronal interface devices could be advanced.